It is known that dietary restriction will prolong life expectancy in aging rats and mice. The purpose of this study is to determine the effects of five dietary programs, consisting of various degrees of protein restriction, in mice. The following indices have been followed: body weight, organ weights, life expectancy, kidney catalase activity, liver aldolase activity, lymphocyte proliferative response to selected mitogens and allogeneic lymphocytes, and serum antibody titer and in vivo plaque assay of immunologically committed spleen cells to sheep red blood cells. Presently the mice are nearing the 50% mortality point, and the various assays enumerated above are being performed.